The present invention relates to stepping-type magnetic mechanisms and, more particularly, to control rod drive mechanisms of the magnetic-jack type.
Commercial nuclear power reactors are typically controlled by the movement of one, or groups of several, neutron-absorbing control rods into or out of the reactor core. A variety of electromechanical devices have been employed or proposed for driving the control rods but two types are most often used today. In the first type, the control rod drive shaft has a plurality of notches extending along its upper portion. A plurality of latch members are actuated in sequence to engage, hold, lift (or lower), and reengage the notches, to withdraw (or insert) the control rod vertically with respect to the core. In the other type, the upper portion of the control rod drive shaft is screw-threaded in mating engagement with a nut member, which is in sequentially rotated to either lift or lower the control rod with respect to the core. In both types of control rod drive mechanisms, the sequential actuation of the latch members or nut rotations, is accomplished step-wise by electromagnetic inductance devices.
Control rod movement usually results in significant reactivity changes in the core, which can increase or decrease the gross power level and affect the local power generated by the individual fuel elements within the core. The accuracy of control rod position indication is an important determinant of the maximum power level at which the reactor can be legally operated. Moreover, if the control rods cannot be controlled precisely enough to assure accurate location in the core and accurate positioning relative to each other, significant operating limitations on the power plant are imposed by law. Operators of nuclear plants thus have a strong incentive to install control rod drive systems that are reliable and rugged, yet maintain maximum accuracy under all normal environmental conditions.
Electromechanical devices of the type described above, have required complex timing schemes and extensive need for calibration, maintenance, and setpoint determination. Even with the best of care, however, power plant operators have lost valuable time and suffered considerable expense when the actual position of the control rods differed signficantly from the positions they should have been in had the drive mechanisms operated flawlessly. For example, position indication was typically inferred from the electrical pulse signals issued to the latch coils. If the latch did not move despite actuation of the coil, the operator's position indication would show an increment in the rod position, even when rod remained stationary. When accumulated over a period of continuous plant operation, such occasional misoperation nevertheless reduced the reliability of the control system, and power plant operation generally.
U.S. Pat. No. 4,125,432, "Drive Mechanism Nuclear Reactor Control Rod", discloses an improvement to earlier drive mechanisms, whereby actual mechanical motion of the control rod gripper latch is detected and utilized in the control and monitoring logic. Previous systems relied upon detection of the latch motion actuation current to infer rod motion. In essence, the method of the '432 patent included the steps of sensing the current level in the drive coil that operated the latch armature, then comparing the waveform of the coil current to at least one waveform characteristic of the coil when the latch is at rest. Thus, the start of latch movement was evident from the deviation of the coil waveform from the waveform characteristic of the stationary latch, and detection of the end of latch movement was evident from the resumption of a coil waveform similar to that characteristic of a stationary latch.
A number of control system improvements potentially available from the teachings of the '432 patent were recognized at that time. Coil lifetime could be improved because the peak current could be limited to that required to cause the latch to start moving; excessive latch wear and coil burnout were thus prevented. By directly monitoring the actual latch movement, individual movements could be accumulated to determine actual rod position in the core. Also, incipient failures could be detected by determining whether the elapsed time between the application of drive voltage and the receipt of the indication that the latch was moving, exceeded a known limit. Finally, the indication of failure of one latch to move as required, could be used to prevent actuation of the other latches associated with the drive mechanism, thereby assuring that the control rod is supported at all times.
Although many advantages relative to the prior art were theoretically available from the invention described in the '432 patent, the practical implementation has not been easy. Two significant difficulties have been encountered. First, the "characteristic waveforms" to which the coil current waveforms are to be compared, are unique to each of several kinds of coils on each drive mechanism, and they differ from mechanism to mechamism. One nuclear reactor may have up to one hundred such mechanisms. Also, the "characteristic waveform" changes in time as the mechanical components of the mechanism are affected by wear and temperature variations. A second obstacle to implementation has been the effect of electromagnetic interference (EMI) on the detection and accurate representation of the current waveforms associated with the coils. The difficulty of accurately determining the coil waveform, coupled with the large number of constantly changing and subtley different "characteristic waveforms" serving as references, presented a considerable obstacle to the design of a cost-effective control rod drive and monitoring system in which all the advantages described in the '432 patent could be achieved.
One of the advantages available from the invention set forth in the '432 patent was, however, implemented in the form of a Gripper Engagement Monitor (GEM) developed by Combustion Engineering, Inc. for use at Songs Units 2 and 3, of the Southern California Edison Company, and at the St. Lucie Number 2 Unit of the Florida Power & Light Company. The GEM circuit detected mechanical motion of the upper and lower grippers by electronically monitoring the gripper coil current waveforms, and provided a gripper interlock such that one of the two grippers would always engage the control rod. When a failure of a gripper to engage was detected, the normal disengagement of the opposite gripper was prevented. Upon such failure, the preprogrammed operating sequence of the control rod timer board was interrupted and the operator recognized that corrective action would be required. The GEM circuit was not used as a controller, but merely as a protection device to notify the operator of a particular malfunction (i.e., failure of a latch to engage).
Although the GEM circuit offered modest improvement relative to the control rod monitoring systems then available, it did not provide most of the advantages available from the invention of the '432 patent. The problems discussed above relating to the large number and wide variety of current wave forms associated with the multiplicity of control rod drive mechanisms for a typical nuclear reactor, as well as the EMI effects, were not overcome by the GEM circuit. By means of a hardwired (operational amplifier) circuit, the GEM could detect latch engagement only for the subset of required latch movements that produce a coil current waveform having a very pronounced shape irregularity, or "glitch", during latch movement. These waveform types are associated with latch engagement, but not with the control rod lifting, or latch pulldown motions which are also required to be performed during the course of a complete latch sequence cycle. Thus, the GEM circuit could not be used to improve timing control and to monitor the satisfactory operation of all the coils associated with the full control rod drive system.
Accordingly, although the invention of the '432 patent was a significant advance in the state of the art at the time such invention was made, and it provided the underpinnings for the GEM circuit which implemented some of the advantages available therefrom, a complete and cost-effective implementation was still needed.